To perform accurate measurements with optical tweezers, it is necessary to calibrate some experimental parameters and even before that study the active components of the optical tweezers setup. The parameters I want to calibrate: Stiffness; which measures the force that is exerts on the trapped bead, and Sensitivity; which measures the relative position of bead within the trap. The components I want to study: Laser, AOM (acoustooptic module), microscope objective, z and x piezo, lens and mirrors, QPD (quadrant photo diode), and electronic filters. It is necessary to characterize these components first because not only they affect the experimental parameters but also reduce the chances to see something unexpected in data later on.

Contents

Characterization

I characterized most of the active components time to time in the setup. The characterization also helped me in designing this tweezers.

Laser

I use two laser: 1064nm 2W ND:YAG crystalaser for tweezing ans 633nm 2mW He-Ne for surface detection and alignment purposes. It was important to know some facts about IR laser. I did not do a rigorous study (because it was not necessary), just specified few parameters like power output, beam waist and its location, polarization, beam mode profile and beam propagation factor. I used this information to design the tweezers expansion optics. The laser specification are given in the following link:

AOM

Acoustooptic modulator is used to modulate the laser intensity in the trap. So it is second most important component of the tweezers. AOM has two components: AOM driver and AOM module, I characterized both of them. The specifaction are given in the link:

I use 1st order diffraction beam from AOM to feed the tweezers. NI-DAQ controlled by feedback main96 labview v7.1 program controls the AOM through analog input voltage to AOM driver. So there is a relationship between the analog input voltage and output laser power in 1st order diffraction beam. Unfortunately this relationship is not linear, it is some odd function (characteristic curve; see the second link). Once i know the curve i can calculate the laser power in the trap at particular input voltage. This information is very useful while calculating the stiffness from the cutoff frequency.

It is absolutely unpractical to measure the laser power in the trap before every power-spectrum data is acquired to calculate the stiffness (for stiffness I need cutoff frequency from power-spectrum and laser power in the trap at which the spectrum is acquired) . So, it is done in advance: I record the laser power after the water-immersion objective for RF-input voltage of 1.3 to 4.9 in .2 volts increments. I put a water droplet on the objective and record the power with a power meter directly. Reflection loss at water-air interface is still less then 2%.

Microscope objective

Objective is another very important part of the tweezers. I am using Olympus UPLANSAPO (UIS 2) water immersion IR objective. The objective gives a maximum spot size of 760nm with a Rayleigh range of 567nm. The objective has 55% transmittance (55% of the input laser power makes through) and it has a collar for cover glass correction (spherical aberration gets worse with the depth in the sample, but i do not have to worry about it while doing power spectrum or DOG). The full details of the objective are available here: